1. Field of the Invention
This invention relates to a light emitting element formed of a group III nitride-based compound semiconductor and, in particular, to a group III nitride-based compound semiconductor light emitting element that is produced by lifting off a sapphire substrate, as a provisional substrate, to have an excellent flatness at the lift-off interface. Also, this invention relates to a method of making the light emitting element.
2. Description of the Related Art
A light emitting element formed of a group III nitride-based compound semiconductor, e.g., GaN is known. A substrate on which to grow the GaN is typically a sapphire substrate which is readily available and processable.
A GaN-based semiconductor layer is formed through a GaN or AlN buffer layer on the sapphire substrate to have a GaN-based light emitting element with a stable crystalline quality. Since the GaN-based light emitting element is adapted to emit a bluish light, it has been used often as a light source to generate white light. In connection with this use, the light emitting element is required to have higher light extraction efficiency.
A factor to determine the light extraction efficiency of the light emitting element is the refractive index of materials to compose the light emitting element. The GaN-based semiconductor layer has a refractive index n of about 2.4 and the sapphire substrate has a refractive index n of about 1.7. Depending on incidence angle of light emitted from inside of the light emitting element, total reflection may occur at the interface of the GaN and the sapphire substrate. Thereby, the reflected light may be absorbed in the light emitting element without being radiated outside to cause a light loss. A known solution to this problem is to lift off the sapphire substrate.
A lift-off method for the sapphire substrate is conducted such that, after a GaN film is grown on a sapphire substrate with a TiN film formed thereon, the sapphire substrate is separated from the GaN film by being heated in a microwave heating device to have a GaN crystal substrate (See JP-A-2004-269313).
However, the above method has a problem that, since the separation is performed by heating the interface of the GaN and the sapphire substrate, the GaN crystal obtained may be damaged near the interface by the heating separation to cause a reduction in light emitting efficiency.
Another lift-off method can be conducted such that the sapphire substrate is separated from the GaN film by irradiating selectively a laser light to the interface of the GaN and the sapphire substrate. However, the same problem applies to this method. Furthermore, in the sapphire lift-off method by the laser irradiation, the laser has to be scanned repeatedly. Thus, the number of steps thereof increases to lower the productivity.